well it certainly sounds like some sort of electrical problem.
thing is they changed wiring in these units over the years so without a serial number it's going to be hard to track down. Maybe you can call someone on location and get them to look on the data plate?

if it is a drive issue the controller may have a MIL light on it, when it's in the fault condition it may flash a code. That's one of the first things i would check if it were me.